This rewrites ArgPromotion to be based on offsets rather than GEP structure. We inspect all loads at constant offsets and remember which types are loaded at which offsets. Then we promote based on those types.
This generalizes ArgPromotion to work with bitcasted loads, and is compatible with opaque pointers.
This patch also fixes incorrect handling of alignment during argument promotion. Previously, the implementation only checked that the pointer is dereferenceable, but was happy to speculate overaligned loads. (I would have fixed this separately in advance, but I found this hard to do with the previous implementation approach).